Uranium can be recovered from commercial grade wet process phosphoric acid by an extraction-reductive stripping process. In this process, phosphoric acid solution is contacted, generally in a multistage, counter-current extractor, with an organic extractant solvent composition having an affinity for uranium values. After extraction, two phases are formed, namely an aqueous acid raffinate phase and an organic phase rich in uranium values. Then, the organic phase is stripped of its uranium content, generally in a multistage countercurrent reductive stripper, and the stripped organic solvent is returned to the extraction system.
This stripping is accomplished by reducing the uranium to the +4 state with a reduced strip acid solution containing a high concentration of ferrous iron, +2 state, in phosphoric acid. Heretofore, elemental iron has only been added to the aqueous acid raffinate in a separate reduction means, to increase ferrous iron concentration, before the strip acid solution is fed to the reductive stripper, as taught by Hurst et al., in U.S. Pat. No. 3,711,591, and Sundar, in U.S. Pat. No. 4,002,716. In the reduction means of these processes, the following primary reaction occurs: Fe.sup.o +2H.sup.+ =Fe.sup.+2 +H.sub.2.
In the process of reducing the uranium in the stripper, the iron +2 in the strip acid is oxidized to ferric iron, +3 stage. Additional iron +2 is also oxidized to iron +3 by any oxidant that is carried over with the extractant solvent and by any air that it contacts. The presence of iron +3 in the stripper from these various sources can reduce the ability of the strip acid solution to remove uranium in the strip step. Iron +3 can also precipitate as either Fe.sub.3 NaH.sub.8 (PO.sub.4).sub.6, Fe.sub.3 KH.sub.8 (PO.sub.4).sub.6 or Fe.sub.3 (NH.sub.4)H.sub.8 (PO.sub.4).sub.6, depending on the availability of sodium, potassium or ammonium ions. Thus, in order to maximize operation and control of the reductive stripper, the Fe.sup.+2 concentration must be maximized.